Although such cells can be used to obtain high energies, they suffer from relatively poor stability by virtue of their reactive anodes being corrosible in the alkaline or saline electrolyte, and in particular they suffer from:
loss of cell capacity during storage by virture of the anode dissolving partially in the electrolyte; and
considerable quantities of gaseous hydrogen being continuously evolved, causing electrolyte to be ejected and causing the cell to swell, and sometimes even to burst.
These unwanted phenomena occur with increased vigor with rising temperature, for example when cells are stored in a hot country.
In order to mitigate these drawbacks and to stabilize a cell of the above type, in particular a cell having an alkaline electrolyte, proposals have been made to stabilize such cells either by incorporating mercury in a zinc anode (thereby forming an amalgam) or else by adding various organic compounds. Proposals were initially made to incorporate a certain percentage of mercury, e.g. 5% to 15% by weight relative to the active metal when the active metal is zinc, and this is applicable in particular for cells having an alkaline electrolyte.
Mercury has the property of increasing the hydrogen overvoltage of the metal in which it is incorporated (in particular when its concentration is high) by inhibiting the proton reduction reaction.
Unfortunately, mercury and more particularly mercury compounds are toxic. This requires strict safety precautions to be taken during the industrial manufacturing process, and it also means that waste cells discarded after use constitute a source of dangerous pollution.
It is thus advantageous to avoid using mercury or to use as little mercury as possible for ecological reasons. In addition, regulations are reducing the allowable amounts of mercury to lower and lower levels.
However, if the amount of mercury drops to less than about 3% by weight relative to the amount of zinc in the anode, adequate stability is not obtained in the resulting alkaline electrolyte battery.
Proposals have also been made to add various organic compounds to a cell having a zinc, aluminum, or magnesium anode in order to improve stability. Such compounds include oleic diethanolamide, monolaurylether, and various polymers of ethylene oxide, and they replace mercury either completely or partially.
Unfortunately, such compounds do not provide adequate stability on their own, and they are often difficult to use or of low efficiency.
For ethylene oxide polymers, particular mention may be made of U.S. Pat. No. 3,847,669 in the name of Union Carbide, which proposes adding an ethylene oxide polymer to a zinc anode containing less than 8% mercury, with the polymer being selected from the group constituted by di-, tri-, and poly-ethylene glycols having an average molecular weight in the range 190 to 7,000 alkyl ethers thereof, and alkanoate esters thereof, in order to improve the stability of cells having such anodes. Said Union Carbide patent describes using alkyl and alkanoate radicals having 1 to 4 atoms of carbon.
In French patent application number 84 10632 filed July 4th, 1984 and entitled "A method of stabilizing electrochemical primary cells having reactive anodes made of zinc, aluminum, or magnesium, and anodes for such cells stabilized by said method", the present Assignee has also described a method of stabilizing primary electrochemical cells including at least one reactive anode made of metal taken from the group consisting of zinc, aluminum, and magnesium, and characterized by adding at least one perfluorinated organic compound of the ethoxyl fluoroalcohol type to said electrode in a concentration lying in the range 0.01% to 1% by weight relative to the weight of the metal.